Diamond is widely used as tools, grindstones, substrates for electronic devices, etc.
For all of these uses, the realization of large diamond substrates is desired. With polycrystal diamond substrates, large substrates exceeding 2 inches in size are already being produced. On the other hand, single-crystal diamond substrates are obtained by slicing a natural or synthetic single-crystal diamond by a method such as cleavage, sawing, laser cutting, or the like, and polishing the diamond, as required; however, large single-crystal diamonds are very expensive when they are natural. Further, when single-crystal diamonds are produced according to a high-temperature high-pressure synthesis method, the synthesis requires an extremely long time, and the yield decreases as the crystal size increases. Therefore, the size is limited to about 1×1 cm. For practical purposes, crystals with a size of up to about 5×5 mm are often used.
With the progression of high-rate growth techniques, the diamond growth technique by a vapor-phase synthesis method has enabled the growth of a diamond at a very high growth rate exceeding 100 μm/h (for example, Non-Patent Document 1). The synthesis of a bulk crystal with a thickness exceeding 1 cm has also been realized (for example, Non-Patent Document 2). Moreover, the vapor-phase synthesis methods have a feature in that the area on which crystals can be grown is easily expanded by enlarging diamond growth apparatuses. This enables growth on large substrates with a high yield by controlling the synthetic conditions and types of impurities to be added.
Hence, large single-crystal substrates can be produced at a high growth rate and a high yield by epitaxially growing a bulk crystal, by a vapor-phase synthesis method, on a large single-crystal substrate obtained from a large bulk crystal synthesized by a high-temperature high-pressure synthesis method, and then slicing the resulting bulk crystal. In this case, because the large crystal used as a seed is expensive and valuable, separation of the large single-crystal substrate from the vapor-phase synthetic diamond in a reusable form is strongly desired.
Furthermore, when single-crystal plates are sliced from the large bulk crystal synthesized by a vapor-phase synthesis method according to the above-described method, a cutting method such as laser cutting or sawing necessitates a cutting margin of several hundred microns with an increase in substrate size. This cutting margin corresponds to a thickness comparable to that of one semiconductor wafer. Thus, a cutting method that minimizes the cutting loss is required.
Parikh et al. have shown a method that potentially meets the above-described requirements, wherein oxygen ions accelerated to a high energy level of several megaelectron volts are implanted into a diamond to form a non-diamond layer, and then the non-diamond layer is removed by annealing in an oxygen atmosphere, thereby separating a single-crystal film with a thickness of the order of about microns from the substrate (see, for example, Non-Patent Document 3). However, as etching proceeds farther inside, the supply of oxygen that passes through the etched non-diamond layer decreases, thereby reducing the etching rate. For this reason, the size of the diamond used is as small as about 4×4 mm. If a sufficient amount of oxygen ions are implanted beforehand, oxygen can also be supplied from the inside of the crystal, allowing the etching of the non-diamond layer to proceed farther. However, because the ion dose increases, the surface layer of the diamond is damaged by radiation.
Marchywka et al. have proposed a method wherein a diamond film is epitaxially grown on an ion-implanted diamond substrate, after which electrochemical etching is performed by applying a direct-current (DC) voltage to remove the non-diamond layer formed by ion implantation, thereby separating the diamond film from the substrate (see, for example, Patent Document 1). Even in this case, however, as the substrate size increases, the etching rate inside the crystal decreases, thus requiring a very long period of time for etching. For this reason, the size of the diamond used is limited to from 2.5×2.5 mm to 4×4 mm, and application to substrates with a size exceeding this range is practically difficult.
Yamamoto et al. have proposed a method wherein a stacked film of a semiconducting diamond layer and an insulating diamond layer is epitaxially grown, or an insulating diamond layer is made conductive by ion implantation, and subsequently, the semiconducting diamond layer is subjected to electrochemical etching, or the ion-implanted layer is subjected to electrochemical etching or electric discharge machining, thereby separating the insulating diamond film (see, for example, Patent Document 2). This method also has the above-mentioned problem of electrochemical etching. Additionally, to form a semiconducting diamond layer necessary for electrochemical etching or electric discharge machining, it is necessary to form a layer doped with an impurity. A plurality of steps are thus required to ensure a satisfactory purity of the insulating diamond layer to be separated.
Patent Document 1: U.S. Pat. No. 5,587,210
Patent Document 2: Japanese Unexamined Patent Publication No. 2005-272197
Non-Patent Document 1: “The effect of nitrogen addition during high-rate homoepitaxial growth of diamond by microwave plasma CVD”; A. Chayahara, Y. Mokuno, Y. Horino, Y. Takasu, H. Kato, H. Yoshikawa, and N. Fujimori; Diamond and Related Materials 13 (2004) 1954-1958Non-Patent Document 2: “Synthesizing single-crystal diamond by repetition of high rate homoepitaxial growth by microwave plasma CVD”; Y. Mokuno, A. Chayahara, Y. Soda, Y. Horino, and N. Fujimori; Diamond and Related Materials 14 (2005) 1743-1746.Non-Patent Document 3: “Single-crystal diamond plate liftoff achieved by ion implantation and subsequent annealing”; N. R. Parikh, J. D. Hunn, E. McGucken, M. L. Swanson, C. W. White, R. A. Rudder, D. P. Malta, J. B. Posthill, and R. J. Markunas; Appl. Phys. Lett. 61 (1992) 3124-3126